The heart is an organ made up mostly of muscle tissue that is responsible for circulating oxygenated blood through an organism's blood vessels using regular, repeated contractions. To facilitate the flow of blood in one direction into the heart, through its various chambers, and out to the rest of the body, the mammalian heart has four valves: the tricuspid valve, the pulmonary valve, the mitral valve, and the aortic valve. In some cases, one of the valves may not function properly, such as due to a congenital defect or disease. The mitral valve, for example, which is responsible for allowing blood to flow from the left atrium of the heart to the left ventricle of the heart, in some cases may not fully open (known as stenosis). As a result, the amount of blood flowing through the heart is decreased, which can lead to serious complications such as atrial fibrillation, blood clots, and lung congestion, heart enlargement, and heart failure, among others. In other cases, the mitral valve may not fully close (known as regurgitation or leakage). In these cases, blood from the left ventricle may leak back into the left atrium, which may lead to problems such as atrial fibrillation, blood clots, infections, and heart failure, among others.